Magnetic heads are being designed smaller and with magnetic properties that allow them to operate at higher frequencies. Magnetic heads are miniaturized to accommodate the small track widths and high bit densities of advanced magnetic media. High frequency operations of magnetic heads are required for faster data transfer. It is desirable to make pole tips of magnetic heads that are a micron or smaller and that can operate efficiently at high frequencies.
Laminated magnetic structures are made of multiple magnetic layers that are separated by non-magnetic layers, referred to as spacer layers, or of alternating magnetic layers with unique properties. The benefits of laminated magnetic structures over single component magnetic structures for application in magnetic read and write devices is well documented; for an early example see Lazzari et al., "Integrated Magnetic Recording Heads", IEEE Transactions of Magnetics Vol. Mag-7, No. 1, March 1971, pp. 146-150.
Laminating magnetic layers reduces eddy currents and eliminates closure domains in a magnetic device. R. F. Soohoo et al., "Switching Dynamics in a Thin Film Recording Head", IEEE Transactions of Magnetics Vol. Mag-18, No. 6, pp. 1128-1130, November 1982, considers the case of a single-turn thin-film head of width (W), wherein (W) is the width of the head perpendicular to the track direction. In the quiescent state, the head is comprised of multiple main domains aligned along the easy-axis-direction within the head, where the easy direction is parallel to the width of the head. In addition, there will be closure domains at the edges of the head with magnetizations aligned normal to the recording track along the hard axis direction. These closure domains occur to compensate for a large demagnetization field that would otherwise be present at the edges of the device. When a field is applied along the hard-axis-direction for the purpose of magnetizing the device, the main domains will respond to the field by rotation of their moments towards the field direction, whereas the edge domains will respond by wall motion. Wall motion reduces the responsiveness of the head. Magnetic structures with closure domains also have greatly reduced coercivities and increased remanences; none of these properties are desirable in magnetic devices operating at high frequencies. Further, the inefficiencies that result from closure domains become significantly more important with narrower head structures.
Magnetic devices that operate efficiently at high frequency magnetization fields, 1 MHz or greater, require that magnetic moments of the magnetic devices be aligned perpendicular to the magnetization gradient. In the case of magnetic heads, the magnetic moments need to be parallel with respect to the magnetic storage medium. This preferred orientation is referred to as an easy-axis-state. To optimize the efficiency of a device, the device magnetization needs to be fixed into the easy-axis-state weakly such that the quiescent magnetization of the device is the easy-axis-state, and yet the magnetic moments of the device respond rapidly to an applied magnetic field for the application at hand. Balancing quiescent magnetization of a device to the easy-axis-state and maintaining sufficiently high coercivity so that the magnetic moments efficiently flip in response to an applied magnetic field is one of the main challenges in designing magnetic structures. In U.S. Pat. No. 5,132,859 to Andricacos et al., and U.S. Pat. No. 5,157,570 to Shukovsky et al., laminated magnetic structures made with magnetic layers of high anisotropy (H.sub.k) coupled to magnetic layers of low anisotropy (H.sub.k) and low coercivity (H.sub.c) are shown to have efficient read and write operations. In U.S. Pat. No. 5,239,435 to Jeffers et al, U.S. Pat. No. 5,018,038 to Nakanishi et al., and U.S. Pat. No. 5,313,356 Ohkubo et al., there are other examples of spacer layer applications for higher frequency operations of head devices and higher density recording discussed. Some of the problems with magnetic heads operating at high frequencies are addressed through implementing laminated magnetic structures. However, laminated magnetic structures using materials and methods in prior art are unsatisfactory to eliminate difficulties encountered in operations of devices at high frequencies when the devices are in the micron or sub-micron size regime.
Forming the pole tip of the magnetic recording head from a laminated structure with alternating magnetic and non-magnetic sub-layers helps reduce closure domains because there is a mechanism for flux closure in the magnetic structure. Flux closure is achieved because alternating magnetic sub-layers have their magnetic moments oriented in an alternating fashion along the easy-axis-direction. Laminated magnetic head structures still, however, have small closure domains and there is directional curling of the magnetic moments near the edges of the laminate. The edge curling region is a significant portion of a pole-tip area when the pole tip is made very small, on the order of microns. In order to keep this region to a small fraction of the total width of the pole-tip, the thicknesses of magnetic sub-layers and non-magnetic sub-layers must be reduced as the total width of the pole-tip is reduced.
Theoretical predictions for required spacer layer thicknesses of non-magnetic transition metal spacer layers to achieve the preferred easy-axis-state are detailed by Slonczewski et al. in "IEEE Transactions of Magnetics Vol. 25, No. 3, May 1988, pp. 245-254. For example, a magnetic head with a laminated pole-tip structure formed from permalloy with a width of half of a micron, will require a laminated structure with magnetic sub-layers 50 Angstroms thick and spacer layers 4 Angstroms thick. For typical materials used in prior art, it is very difficult to fabricate spacer layers which are this thin and that do not contain deleterious pin-holes and defects in the spacer layer structure. Magnetic layer structures that have pin-holes and defects in spacer layers act as if they are not laminated due to strong magnetic coupling between the magnetic layers.
It has been discovered that non-magnetic transition metals, when used as spacer layers, will couple magnetic layers in a laminated magnetic structure. The nature of this coupling has been the subject of extensive theoretical discussion and experimentation; see S. S. P. Parkin et al., "Oscillations in Exchange Coupling and Magnetoresitance in Metallic Superlatice Structures: Co/Ru, Co/Cr, and Fe/Cr", Phys. Rev. Lett. Vol. 64, No. 19, 1990, pp. 2304-2307; S. S. P. Parkin et al., "Spin Engineering: Direct Determination of the Ruder-Kittel-Kasuya-Yosida Far-field Range Function in Ruthenium", Phys. Rev. B., Vol. 44, No. 13, 1991, pp. 7131-7134; S. S. P. Parkin et al., "Systematic Variation of the Strength and Oscillation Period of Indirect Magnetic Exchange Coupling through 3d, 4d, and 5d Transition Metals", Phys. Rev. Lett., Vol. 67, No. 25, 1991, pp. 3598-3601; S. S. P Parkin et al., "Oscillatory Exchange Coupling and Giant Magnetoresistance via Cu-X Alloys (X=Au, Fe, Ni)", Europhys. Lett., Vol. 24, 1993, pp. 71-76; P. J. H. Bloemen, "Interlayer Coupling in Co/Os Multilayers", J. Magn. Magn. Mater. Vol. 121, 1993, pp. 306-308: and M. van Schilfgaarde et al., "Theory of Oscillatory Exchange coupling in Fe/(V,Cr) and Fe/(Cr,Mn)", Phys. Rev. Lett., Vol. 74, No. 20, 1995, pp. 4063-4067. It has been established that several elements of non-magnetic transition metals spacer layers in a magnetic device will generate anti-ferromagnetic (AFM) coupling in the limits of ultra-thin spacer layers (2 to 50 Angstroms). These coupling strengths are oscillatory with respect to spacer layer thickness and can be very strong. Parkin et al. in U.S. Pat. No. 5,341,118 describes multi-layered magnetic structures with non-magnetic spacer layers that strongly couple ferromagnetic layers and exhibit high magnetoresistance (MR) in fields that are sufficiently low to be useful in MR sensor construction. The AFM coupling is used to pin adjacent magnetic layers through thin transition metal spacer layers with thicknesses corresponding to the maximums of the oscillatory AFM coupling functions, whereby the magnetic permeability is essentially zero in the absence of an applied field and remains essentially zero in the absence of strong externally applied magnetic fields. Gill et al. in U.S. Pat. No. 5,701,222 and Parkin in U.S. Pat. No. 5,585,986 describe further MR sensor applications for strongly AFM coupled magnetic structures. However, for some applications what is needed is weak AFM coupling, whereby the magnetic structures exhibit non-zero permeability value in the presence of applied magnetic field. Examples, include very narrow track width head pole tips and flux extenders for read heads where the goal is to eliminate or reduce closure domains while maintaining a useful effective permeability values. What is needed is a method for making multi-layer magnetic structures containing non-magnetic transition metal layer that have been engineered to provide AFM coupling strengths useful for applications such as miniaturized pole tips and flux extenders.